The present invention relates to a liquid crystal panel for displaying an image, and more particularly to passivation which is useful for an active matrix addressed liquid crystal image display incorporating a switching device for each pixel.
In an active matrix addressed liquid crystal panel, it is difficult to drive all the liquid crystal cells under the same condition because of its complicated device structure and so is because it is prone to generate a phenomenon wherein a displayed image glimmers. As is well known, this phenomenon (also called "flicker") occurs when the matrix addressed liquid crystal panel is observed aslant, or a large amount of D.C. component is contained in a driving signal.
In order to reduce the flicker, two techniques have been proposed. One is to fabricate the components constituting the panel, i.e. liquid crystal cells formed of insulated-gate transistors and storage capacitors, with high accuracy so that all the liquid crystal cells can be driven under the same condition. The other is to reduce the flicker visually, more specifically, to drive the adjacent liquid crystal cells in phases opposite to each other so that the flicker is not observed on the entire panel.
Both techniques, however, have the following drawbacks. In the former, the condition of fabricating an active substrate and assembling the panel is strictly set and also a large storage capacitance is required; this will reduce the fabricating yield and the numerical aperture. On the other hand, in the latter, the flicker can be reduced apparently, but the counter electrode is A.C. driven under the condition held at a constant voltage so that the signal voltage will be boosted. This will increase the minute fluctuated D.C. voltage component among the liquid cells which will cause the flicker, so that the liquid crystal is deteriorated and becomes brown over use for a long time, thus providing poor image quality.
Essentially, if as shown in FIG. 6, an orienting organic thin film 18 completely insulates the surfaces of a signal line 12, a drain wiring 22, a pixel electrode 14, etc. a D.C. current will not flow into a liquid crystal cell 13 consisting of the pixel electrode 14, a counter electrode 15 and a liquid crystal layer 16, and so the liquid crystal layer 16 will not also be deteriorated. The orienting film 18 alone, however, cannot completely insulate the surfaces of the signal line 12, the drain wiring 22, the pixel electrode 14, etc. because the orienting film 18 is thin (about 0.1 .mu.m), is liable to contain pinholes because it is applied generally by offset printing, and is cured or thermally set at a comparatively low temperature of 300.degree. C. or lower so that the active devices and the colored layer 17 of a color filter 9 are not thermally destroyed; it is difficult to prevent the liquid crystal layer 16 from deteriorating more or less. Particularly, the D.C. component is liable to flow between the signal line 12 and the counter electrode 15 because the signal voltage continues to be externally applied to the signal line 12. Thus, in order to prevent the liquid crystal layer from becoming deteriorated, as shown in FIG. 7, it is proposed to coat the entire surface of an active substrate 2 with a transparent insulating film of Si.sub.3 N.sub.4 having a thickness of 0.5 .mu.m in place of the thin orienting film.
This technique of depositing the thick passivation film 23 on the entire surface, however, is also not necessarily preferable since it lengthens the fabricating process, and the presence of the excess insulating layer on the pixel electrode 14 reduces the effective voltage to be applied to the liquid crystal layer 16. Although the problem of the reduction of the effective voltage can be solved by selectively removing the passivation film 23 on the pixel electrode, a large level difference in the passivation film occurs on the pixel electrode 14 or in its neighborhood. This makes it impossible to regularly rub the orienting film 18 with a dry cloth, so that the orientation of the liquid crystal falls into disorder to produce inverted domains; this eventually reduces the image quality. Further, in order to obtain the passivation film 23 having good film quality, the active elements are required to have strict heat resistance; this makes it difficult to ensure the characteristic of the insulated gate transistors.